BRITISH TOLL OF 50 FEARED IN ARGENTINE ATTACK

By R.W. APPLE Jr., Special to the New York Times

Published: June 11, 1982

LONDON, June 10—
As many as 50 soldiers and sailors may have been killed in Argentine air attacks on British landing craft Tuesday, unofficial military sources said tonight. They described the episode as the most serious setback suffered by Britain in the war in the Falkland Islands.

For the first time since hostilities began almost 10 weeks ago, the Government refused to disclose casualty figures. Speaking in the House of Commons this afternoon, John Nott, the Defense Secretary, asserted that details of British losses ''could be of benefit to the enemy and put our own men at greater risk.'' His statement lent credence to the belief that the losses were the worst of the war.

At the meeting of Atlantic alliance leaders in Bonn, Prime Minister Margaret Thatcher spoke of ''quite a considerable number of casualties.''

Mr. Nott dismissed a suggestion from a member of the Labor opposition that the Falkland conflict was turning into ''Britain's Vietnam,'' characterizing the war as ''a series of major victories with some setbacks.'' He promised that British troops would ''go forward with another victory very soon'' - a reference, presumably, to the long-anticipated attack on the Argentine garrison of about 7,000 bottled up in and near Stanley, the islands' main settlement.

The minister insisted that the plans of the task force commander, Rear Adm. John F. Woodward, had not been jeopardized by the effects of the devastating Argentine attacks. But he did not specifically say that there would be no delay, and Brig. Tony Wilson, commander of the Fifth Brigade, was quoted in correspondents' dispatches as having said, ''Until we have assessed the situation we do not know how much this will change our plans.''

Mr. Nott told the Commons that the equipment and supplies lost during the attack were being made up from stocks on the islands. Roaring in from behind a ridge on shore, Argentine Skyhawks and Mirages bombed two 5,674-ton landing ships in the estuary at Fitzroy, about 15 miles southwest of Stanley. Each of the ships - the Sir Galahad and the Sir Lancelot, named for knights of King Arthur's Round Table - had carried 250 to 400 troops to the beachhead. Both Ships Are Abandoned

Both were set afire and had to be abandoned, military sources indicated, but it was the Sir Galahad that took the brunt of the attack. With both the ship and oil-covered water surrounding her afire, with ammunition boxes and fuel tanks exploding, hundreds of men were burned. An army doctor at Fitzroy, treating casualties plucked from the burning ship and from the water by daredevil helicopter pilots, said that he had ''never seen anything as horrifying.''

''The flames from the landing craft lit up the night sky,'' a pooled dispatch from British reporters said. ''The narrow inlet was filled with rescue craft. As they were stretchered to a field hospital, some of the casualties were screaming in agony.''

A smaller landing craft was also hit near Lively Island, to the south, and her stern was blown off. Mr. Nott confirmed that in that incident two sailors and four Royal Marines had been killed.

The sole consolation for the British was that Harrier jump-jets reportedly managed during the day to shoot down six or seven enemy jet and to damage four others so severely that they were considered unlikely to reach home. But the Argentine air force has sustained similar losses on several occasions in the past and still come back for more. Undercurrent of Criticism

Although few politicians or military men were willing to voice public misgivings, there was an undercurrent of criticism of Brigadier Wilson for attempting to land his men in daylight without adequate antiaircraft protection. There appeared to have been no destroyers or frigates in the immediate area, and Rapier surface-toair missiles had not yet been installed.

The debacle at Fitzroy also prompted a renewal of complaints about the task force's lack of airborne early-warning radar. The men aboard the ships had almost no warning. A corporal from the Welsh Guards, rescued from the blazing Sir Galahad, said afterward, ''I was told to get down, and the next thing I knew I was on fire, like.''